Two Johns Hopkins professors and a Swiss researcher have been selected to receive the 1978 Nobel Prize in medicine for their pioneering work enabling scientists to break apart the molecules that shape life and reconstruct them in different combinations.

The Karolina Institute, in Stockholm, which selects the winners of the prestigious international prizes, said the work of doctors Daniel Nathans, Hamilton O. Smith and Werner Arber "increased knowledge in this area (which) should help in the prevention and treatment of malformations, hereditary disease and cancer."

Speaking at a hastily arranged press conference here yesterday, Smith and Nathans each spoke of the importance of the other's work.

But the story of both men's achievement, Smith said, "begins predominately with the work of Dr. Werner Arber in the late 1950s and early '60s."

Arber, of Basel, Switzerland, is credited as the discoverer of "restriction enzymes" chemicals that dismantle genetic material at specific points, much the way a cook breaks apart a chicken at the joints for frying.

Dr. Hamilton O. Smith, a 47-year-old professor of microbiology at Johns Hopkins University Medical School here, was cited for having discovered and worked with the first such restriction enzyme, found in a form of an influenza bacterium.

And Nathans, the chairman of the department of microbiology at Hopkins, has been using the enzyme to break up the DNA (deoxyribonucleic acid, which is the long molecule containing the human genetic patter) found in cancer cells.

The application of the prize winners' work best known to the public has been in the field of recombinant DNA research, the highly controversial rearranging of genetic material to create altered or new forms of life.

Recombinant DNA experiments have been attacked by critics who believe that man has no business altering the basic patterns of life, or that they might unleash a virus or bacterium that could alter or destroy life as we know it.

Ironically, Nathans, who began dismantling the DNA of cancer cells in 1970 in an attempt to learn what made them different from normal cells, was one of those scientists who in 1974 urged that research on recombinant DNA move with great caution.

Much of the controversy has since dissipated, as work has gone on at a number of institutions without any problems, and with such clear benefits as the recent production of synthetic insulin in the laboratory.

Smith said yesterday he believes "we've come through a period when (the fear of DNA research) was perhaps overblown, and now we're coming back to a more reasonable position."

Both Smith and Nathans said they learned they had won the prize - the fifth year in a row the medical prize has gone to Americans - when they were beseiged by phone calls yesterday morning from reporters. "It took me a few minutes to recover from the shock," Smith said.

Nathans said he felt "disbelief" as well. "I like to get confirmation before I invest so much emotional energy," he said, commenting that he did not even know the amount of the prize money - $165,000 - to be divided among the three researchers.

Smith yesterday described his career as being "like a cork bobbing on the water, I just drifted here and there, until I found something I like." His interest in molecular biology began with some reading he did to fill the empty hours spent as a ship's doctor during a two year stint in the Navy.

The prize winning discovery was "accidental," he said, as are many major scientific discoveries, and he gave much of the credit for the breakthrough to a student and post-graduate who worked in his laboratory at Johns Hopkins.

Smith said he and Dr. Kent Wilcox, a then student in the lab, were doing work with the DNA in a form of influenza when they "accidentally discovered the restrictive enzyme" in the flu bacterium.

Wilcox was drafted into the army shortly after the discovery was made, and a post-graduate fellow took his place in the laboratory. Asked if Wilcox might be sharing the limelight had he not been drafted, Smith pointed to his seat and said "he might be sitting here."

Nathans, who was doing research in Israel in 1969, said Smith wrote to tell him about the breakthrough and the letter "suddenly sparked my interest that restriction enzymes could be used" to cut up DNA just as digestive enzymes in human bodies break up the protein in food.

"In order to have any hope of understanding the complicated genetic picture," of cancer, birth defects, human development, "we have to break it up one piece at a time," said Nathans.

Nathans is continuing to look at pieces of DNA in cancerous cells in the hope of discovering what triggers the cancerous growth of an otherwise normal cell.

And Smith said he is returning to the work with influenza DNA which was interrupted when he "accidentally" made the discovery that lead to his winning a Nobel Prize.